1. Field of the Invention
The present invention relates to a driving device, and more particularly it pertains to a LED driving device capable of improving the power factor and efficiency.
2. Description of the Related Art
Diode, a semi-conductor element, works like a switch, has played an important role in electronic system.
There are many kinds of diode around our life. One kind of diode can be lighting when being energized. This kind of diode we call it Light Emitting Diode (LED).
The application of LED is quite wide. High bright LED is widely used for traffic light, vehicle indicating light, and braking light. Full-color LED display, composed of red LED, green LED, and blue LED, is also used for stadium and street advertisement, such as the larg LED display at outside of Nasdaq marketing center in Times Square, New York city. The 20-foot high screen, composed of 19,000,000 high bright LEDs, is the largest one in the world.
LED has become an indispensable lighting device today because cell phone and portable electronic products are getting more popular. Experts believe that the LED will replace most light source in the near future due to its advantage of space-less, high lighting speed, and long lifetime.
LED is so widely used that many kinds of driving devices and chips are worked out. FIG. 1A is a diagram showing a current versus voltage relationship of a LED. The relation of the voltage and the current can be represented by an exponential function and the relation is similar to an ordinary diode. When the forward voltage is less than some value Vb, only very small current flows through the LED. When the voltage exceeds some value Vb, the current would raise sharply. The sharp current is forward current of LED. Said value Vb we call it barrier voltage. The Vb value is between 1.5V and 3.5V usually. The semiconductor material and doping level decides the barrier voltage Vb. Besides, the wave-length of the light emitted from a LED also depends on the kind of material, for example, red Led is composed of GaAsP.
The LED light output luminous intensity is proportional to LED current for most operating value of LED current, but the approximation usually over-estimates light output at high current value. A typical curve is shown in FIG. 1B. Actually, the driving devices are designed to provide a constant current for stabilizing light emitted and extending the life of LED.
FIG. 1C is a waveform diagram of voltage and current for explaining the behavior of a LED. If we use AC power source to energize the LED, the light will be emitted during the interval T within the positive part of the AC power source because the voltage level of the AC power source is higher than the barrier voltage of the LED. We could couple a bridge rectifier to AC power source for taking the advantage of AC power source. By applying a bridge rectifier, the negative part of AC power source will be converted to positive. FIG. 2A shows the circuit diagram of bridge rectifier. To get a stable voltage supply, a filtering capacitor can be coupled to. FIG. 2B shows the waveform diagram of voltage and FIG. 2C shows the waveform diagram of current related to FIG. 2B. Further, a constant current circuit is added to keep the constant luminance and color of light emitted. FIG. 2D shows the waveform diagram of current in above situation.
It is important that if there is a capacitor or an inductor in the circuit, the current and voltage will be non-synchronous. FIG. 2E is a waveform diagram of current lagging behind voltage waveform. If the voltage Eac is represented as Em Sin ωt, the current Iac is represented as Im Sin(ωt−Θ). In above situation, the product of voltage and current is not always positive. The power value is calculated as VmIm Cos Θ/2 and it is less than the power of the voltage and current in the same phase VmIm/2. The Cos Θ was called power factor.
There are many patents about LED driving device. For example, U.S. Pat. No. 5,936,599, “AC POWERED LIGHT EMITTING DIODE ARRAY CIRCUITS FOR USE IN TRAFFIC SIGNAL DISPLAYS”, FIG. 3 shows its circuit diagram. According to this diagram, we see a LED array including a number of series connected polarized LEDs Dpair are energized by an AC voltage source coupled to an inductor. Each polarized LEDs including two parallel connected oppositely polarized LEDs. The inductor is taken the place of the resistor which is used to limit the current. The inductor will limit the current with less power loss than the resistor does. To further reduce power loss, a capacitor is coupled to LED array. The capacitor has to be tuned to match up the inductor and the frequency of the AC voltage source. However, the power loss is improved but the power factor is reduced for the existence of the capacitor and inductor. Another disadvantage is that the capacitor and inductor have to be tuned with the frequency of AC voltage source. Besides, the number of LEDs in LED array is dependent on the voltage level of the AC voltage source.
U.S. Pat. No. 5,457,450, “LED TRAFFIC SIGNAL LIGHT WITH AUTOMATIC LOW-LINE VOLTAGE COMPENSATING CIRCUIT”, its circuit diagram is shown as FIG. 4. To avoid the overall light intensity dropping down, a mid-voltage compensation circuit 82 and a low-voltage compensation circuit 84 has been designed. Although this patent can avoid light intensity from dropping down without increasing the power consumption, but driving LEDs with increased current will shorten the life of LEDs.
In addition, a stable voltage source is always used to driving LEDs for increasing the lighting time of LED and a filtering capacitor is used for this purpose. The disadvantages for the existence of capacitor are lowering the power factor and the capacitor has to be changed with the frequency of power source. Besides, the life and stability of capacitor is affected by temperature very heavily so that it makes the driving device unstable.